For example, a power supply apparatus disclosed in patent document 1 is known as a power supply apparatus of, for example, DC-DC converters. The power supply apparatus includes a transformer, a primary semiconductor component, a secondary semiconductor component, and a choke coil. The primary semiconductor component is connected to the primary coil of the transformer, and the secondary semiconductor component is connected to the secondary coil of the transformer. The choke coil is connected to the secondary semiconductor component. These components are mounted on one of the major surfaces of a base plate; the major surfaces extend, i.e. spread, in a two-dimensional direction. Specifically, these components are arranged on one of the major surfaces while extending in the two-dimensional direction. The two-dimensional extending direction of the major surfaces of the base plate will be referred to merely as an extending direction.